Amplifier feedback suppression



Aug. 29, 1939. i F. STRECKER 2,170,878

AMPLIFIER FEEDBACK SUPPRESSION Filed Dec. 3, 1956 INVENTOR FELIX TRECKER ATTORNEY Patented Aug. 29, 1939 UNITED STATES PATENT OFFICE AMPLIFIER FEEDBACK SUPPRESSION Application December 3, 1936, Serial No. 113,970 In Germany December 4, 1935 8 Claims.

This invention relates to an improved amplifier arrangement for echo and feedback suppression.

In echo and feedback suppressors, extremely small response periods are frequently required, in the case of the former particularly when they are located in the immediate vicinity of split connections. The time which a signal requires to travel the distance from the branching point as far as the suppressor is extremely short because of the small length of the intervening path; hence, the echo can be killed sufiiciently only by the aid of suppressors insuring an extremely speedy response.

In the suppressor circuit organization known in the prior art the said severe requirements have not been satisfied in all cases. The conditions become particularly diflicult whenever the suppression is effected by the aid of mechanical relays involving inherently a relatively long response period. In fact, with suppressors of this kind it has not been feasible to diminish the response to a few milliseconds.

Now, according to the present invention, these difiiculties can be obviated by the fact that in the control circuit of the suppressor a screengrid tube is used. It is known that the inner impedance of screen-grid tubes is very high, while the inner resistance to direct current is comparatively low. Examining, for instance, in an echo killer the circuit consisting of the plate circuit of the control tube and the relay winding, it will be seen that the time which is required for the rise of the current in the relay is a func- 35 tion of the value where L and R are the inductance and resistance of the relay coil, and R1 the internal resistance to alternating current of the control tube. Now, inasmuch as this period is an essential part of the total time lag involved in the response action, it follows thatv by an increase of resistance, in other words, by the use of a screen-grid tube, the response time of the suppressor can be substantially decreased over the circuit organizations known in the prior art predicated for their operation upon the use of standard triode tubes.

What is meant by the control circuit of the suppressor in the present connection is the part of the circuit organization which comprises such circuit elements as are required for the suppressor action. Hence, what is primarily involved is the portion intervening between the branchoff point and the point where the suppressor action takes place, though these circuit elements could be situated also in the part of the line preceding the branch-off point.

By the use of a screen-grid tube not only the time of response may be reduced to a considerable extent, but there results in addition the advantage that a certain demand which is important so far as echo killers are concerned, may be satisfied with comparatively simple means. This requirement is that the response time and the time of aftereflect or decay should as far as feasible be independent of the value of the voice voltage which, in turn, is a function of the speech volume of the speaker and the nature of the connection. Moreover, suppressors should be irresponsive to voltages falling below a certain critical value, and this is a point that is of importance in order to prevent actuation of the suppressors as a result of disturbing or stray currents.

The decay time, in other words, the period required by the current or the potential in the suppressor circuit to decline from its crest value to a certain minimum value is a function of the size of the crest value. This time naturally will be so much greater, the higher the value of the peak from which decay takes place to a minimum value. It is, therefore, necessary that the maximum value should be limited and to thus prevent any substantial variations of the decay period.

The response period, in other words, the period which elapses between the arrival of the signal currents at the suppressor up to the occurrence of the suppression is dependent upon the size of the incoming signal voltages, the rise taking place so much more steeply, the greater the supplied alternating current amplitude. Hence, it would also be suitable, with a view to stabilizing the response period, to limit the crest value. If an additional care is taken so that the upper limit will be attained even by comparatively low signal voltages located above the lower threshold, there results a marked independence of the response or building-up period and the decay period so far as the size of the voice voltages is concerned. Referring, now, for a more complete understanding of my invention, to the following detailed description, which is accompanied by a drawing, in which Figures 1 and 2 are curves explanatory of features of my invention, while Figure 3 illustrates diagrammatically a simplified form of my invention and Figure 4 illustrates, in conjunction with Figure 3, a modification of my invention.

If the requirements heretofore set forth, together with the above demand for a lower threshold value are taken into consideration, there results a relationship between the alternating voltage U1 prevailing at the input end and the regulated amplified alternating voltage U2 at the output end of the amplifier comprised in the suppressor arrangement as graphically shown in Fig. 1. Distinction may be made between two threshold values S1 and S2. In the presence of voltages below S1, there is practically no output Voltage U2, while for potentials above S2 the out:

put alternating potential stays practically stable. Since, as pointed out above, the response and the decay periods are a function of the size of the potentials applied to the stopper means, no

changes in these times will occur any longer above the point S2, but for the transition between the two threshold values S1 and S2, dependence of the response and decay periods upon the size: of the input signal voltages holds good as before.

Hence, with a View to insuring efiicient and perfect operation of the stopper or suppressor means there arises the demand that the two threshold values should be located as close to gether as feasible in order that the transition should be reduced. This condition is fulfillable by the use of a screen-grid tube connected below the limiter means as shall hereinafter be explained in more detail by reference to Fig. 2. This shows the dependence of the plate current is. upon the grid voltage 6 of an amplifier tube at which, for instance, by the initiation of grid current fiow the growth of the plate current for positive grid potentials is practically prevented; 2}) denotes the lower limiting value of the plate current which is required in order to insure the stopper action. Hence, this current governs the lower threshold value S1. The upper threshold value S2 lies at about zero grid voltage. The threshold values S1 and S2 will, relatively speaking, be placed at so much greater proximity to each other, the higher the alternating (signal) voltage is in comparison to the full operating range of the tube. If the working point A is. chosen markedly negative by the aid of a sufficiently high grid biasing voltage gv, then a high voltage e1 is required in order that the lower threshold value is may be attained. In other words, high amplification of the signal voltages impressed upon the tube is necessary. Therefore, it is advisable to use an input amplifier tube of the screen-grid type.

Figs. 3 and 4 illustrate exemplified embodiments of the basic idea of this invention. In the case of Fig. 3, the screen-grid tube SV is connected above the rectifier G1 to which the alternating voltage is fed by way of transformer T. In the plate circuit' of the screen-grid tube is connected the relay R Whose contact 1' serves to insure the stopper or suppressor action. The tube is biased by means of battery B to the point e v, as indicated in Figure 2.

Fig. 4 shows the circuit included below the rectifier G1. The output of the rectifiers G1 is applied to the screen grid tube SV of Figure 3 as indicated by dotted lines X X. In other words, the portion of Figure 3 to the right of the dotted line is to be considered added to the right of the dotted line in Figure 4. The signal voltage is first amplified in the screen-grid tube V SV and is then fed to the limiter tube V. Owing to the high resistance Rg in the grid circuit of this tube, and by the adjustment of a sufficiently high negative grid biasing voltage by the aid of the battery B, this tube will be made to operate in a way as indicated in Fig. 2.

What is claimed is:

1. A circuit for echo suppression in a signal line comprising an input circuit, connected to said line, including a full-wave rectifier; a screen grid tube having a control grid connected to said rectifier, and a plate circuit; a relay winding in the plate circuit of said screen grid tube, and means for maintaining the grid of said screen grid tube biased substantially beyond cut-off the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced.

2. In a circuit for echo suppression in a signal line, an amplifier connected to said line; a rectifier; a screen grid tube having a control grid connected to said rectifier and plate circuit; a relay winding in the plate circuit of said screen grid tube, means for maintaining the grid of said screen grid tube biased substantially beyond cutoff, and control means actuated by said relay winding the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced.

3. A circuit for echo suppression in a signal line comprising an amplifier having an input circuit connected to the line, and an output circuit; a limiting circuit including a thermionic discharge tube having grid and plate circuits, said grid circuit being connected to the output of said amplifier; means for maintaining the grid biased substantially beyond cut-oil, a rectifier coupled to the plate circuit of said tube; a screen grid tube having a control grid and a plate, the

control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced.

4. In a circuit for echo suppression in a signal line, an amplifier having an input circuit connected to said line and an output circuit, a limiting circuit including a thermionic discharge tube having a grid and a plate, said grid connected to the output of said amplifier, means for maintaining the grid biased substantially beyond cut-ofi, a rectifier coupled to the plate of said tube, a screen grid tube having a control grid and a plate, the control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to theimpedance of said. suppressor relay winding whereby the operating time lag of said relay is reduced.

5. A circuit for echo suppression in a signal line comprising a high gain amplifier having an input circuit connected to the line, and an output circuit, a limiting circuit including a thermionic discharge tube having grid and plate circuits, said grid circuit including a resistance in series with the grid and connected to the output of said amplifier, means for maintaining the grid biased substantially beyond cut-oil, a rectifier coupled to the plate of said tube, a screen grid tube having a control grid and plate, the control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced 6. A circuit for echo suppression in a signal line comprising a high gain amplifier having an input circuit connected to the line, and an output circuit, a limiting circuit including a thermionic discharge tube having grid and plate circuits, said grid circuit including a resistance in series with the grid and connected to the output of said amplifier, means for maintaining the grid biased substantially beyond cut-01f, a full wave rectifier coupled to the plate of said tube, a screen grid tube having a control grid and plate, the control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced.

7. A circuit for echo suppression in a signal line comprising a high gain amplifier having an input circuit connected to the line and an output circuit, a limiting circuit including a thermionic discharge tube having grid and plate circuits, said grid circuit including a pair of resistances connected in series, the output of said amplifier connected between said resistances, means for maintaining the grid biased substantially beyond cut-01f, a rectifier coupled to the plate of said tube, a screen grid tube having a control grid and plate, the control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay Winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced.

8. A circuit for echo suppression in a signal line comprising a high gain amplifier having an input circuit connected to the line and an output circuit, a limiting circuit including a thermionic discharge tube having grid and plate circuits, said grid circuit including a pair of resistances connected in series, the output of said amplifier connected between said resistances, means for maintaining the grid biased substantially beyond cut-off, a full wave rectifier coupled to the plate of said tube, a screen grid tube having a control grid and plate, the control grid of said screen grid tube connected to said rectifier and an output circuit including a suppressor relay winding connected to the plate of said screen grid tube the plate resistance of said screen grid tube being large compared to the impedance of said suppressor relay winding whereby the operating time lag of said relay is reduced,

FELIX STRECKER. 

